Physical Geography
the climate of the Indian subcontinent can be divided into four subseasons
- a relatively cool period from December to March - a very hot, dry period in April and May - a cooler wet period of the monsoon in June-September - a warmer period during the retreat of the monsoon, accompanied by cyclonic rain
incoming radiation
- absorbed by gases in atmosphere, oxygen and ozone at high levels and carbon dioxide and water vapour at lower levels - radiation is scattered through gas molecules and dust particles - clouds and water droplets reflect radiation, thick clouds can reflect up to 80% of total incoming radiation. - earth surface reflects radiation, with land reflecting more than water
processes other than abrasion can create platforms
- around British coasts, limestone outcrops are characteristically pitted and honeycombed, showing chemical weathering - in tropical waters, chemical and biological factors promote "reef flats" in intertidal zones
glacial control theory coral reefs (Daly)
- coastal shorelines originated during glacial low sea levels and have built up with the sea level rise in postglacial time. - however, borings of many atolls have shown that coral structures exist to depths up to 760m, well beyond the lowest glacial sea levels
what affects sea level change?
- eustatic changes - tectonic changes (isostatic, epeirogenic, orogenic)
negative feedback of flat waves
- flat waves shift sediment forwards, steepening the beach - this increase in gradient will shorten the wavelength and steepen the waves, making the waves more destructive - thus creating an equilibrium between process and form
anticyclonic weather is not uniformly fine
- in winter, some anticyclones give clear skies and low temperatures, and severe frosts may occur - when moist Pm air is present, may form continuous stratus cloud trapped beneath an inversion formed in the subsiding air above - anticyclonic gloom can combine with the products of industrial and domestic combustion to create considerable pollution problem (eg in large cities in Britain) however, most anticyclonic spells in summer in mid-latitude areas are uniformly fine and often give the maximum temperatures of the year
why are Barchan dunes crescent shaped?
- material moved by wind up windward slope - makes lee slope steeper - when too steep, shears, material slips to bottom of slope - horns because sand slides faster on lower mound - rate of advance inversely proportional to height of slip face
variations in composition of atmosphere with height
- mixing and turbulence in the lower regions prevents gases from separating out to form individual layers - at great heights certain heavier gases begin to separate, for example ozone - at the highest points, satellite evidence suggests that the lightest gases like nitrogen separate out (nitrogen, oxygen, helium, hydrogen, where there appears to be no upper limit)
air mass source regions
- occur in regions of relative calm in the general circulation, generally into semi-permanent anticyclones, where air circulation is slow enough for homogenous air mass characteristics to develop - source regions are usually areas where the earth's surface is fairly uniform (deserts, oceans, large ice and snow covered areas)
significance of knickpoint in desert mountain fronts?
- pediment has concave profile, suggesting fluvial action - pediment (slope before cliff) cut by rill action and sheetwash - scarp, however (mountain front) recedes in parallel retreat through weathering and mass movement - therefore, knick shows change in processes, from fluvial to wind/geological
what controls air motion?
- pressure gradient - Coriolis effect - centripetal force, which acts around circulatory pressure systems - frictional force exerted by the earth's surface
what are the conditions for a thunderstorm to form?
- rapid cloud formation and heavy precipitation in unstable air conditions - absolute or conditional instability should extend to great heights, which allows powerful updrafts to develop within towering cumulonimbus clouds which are associated with thunderstorms - the largest storms tend to occur in tropical or warm conditions where the air can hold considerable amounts of water, and are rarer in polar conditions
alternative subsidence theory corals (Darwin)
- small islands have slowly subsided with the general downwarping of oceanic parts of the Earth's crust - thus, originally fringing reefs became barrier reefs and eventually atolls as the land core submerged. - with different rates of subsidence we see coral shorelines at all these stages of development - evidence shows coral bases at a wide variety of depths which suggests subsidence theory is more feasible - however practically all reefs record evidence of eustatic oscillations, in the form of submerged notches and benches, so glacial control may account for some of their features.
negative feedback of beach backwash
- steep waves comb down the beach, moving sediment seawards in their powerful backwash - this reduces the beach gradient, increases the wavelength of the waves, and reduces their steepness - thus creating an equilibrium between process and form
why do rivers create concave erosion?
- waves are propagated horizontally under influence of gravity, will naturally move down slope - abrasion of particles is greater with greater discharge/river water energy. for shorter distances therefore more erosion further down the river? - further down river means more tributaries adding water and sediment
how are offshore bars formed?
- when waves approach a gently sloping coast, friction between the waves and the sea bed causes the waves to break at some distance fro, the coast. - over time, more materials are built up parallel to the coast to form a ridge of sand called an offshore bar. - a body of coastal water may be partially or completely cut off from the open sea by the offshore bar. this body of water is called a lagoon.
Bayhead beach
A crescent shaped beach in a sheltered cove
syncline
A downward fold in rock formed by compression in Earth's crust
maritime air mass
A humid air mass that forms over oceans
cuspate bar
A triangular, seaward projection of the beach, formed when material has drifted along opposing directions. For example, Dungeness in Kent is a cuspate bar where dominant breakers have built up a series of shingle storm ridges over the period of many centuries.
granite
A usually light colored igneous rock that is found in continental crust
how do clouds produce rain? (Bergeron theory)
Bergeron theory: Production of raindrops depends on the coexistence of water and ice in clouds at temperatures well below freezing. Clouds often contain supercooled water droplets down to temperatures of -40 degrees and have freezing nuclei required to make ice crystals. In a cloud were
polar air masses
Form from north of 50 degrees north latitude and south of 50 degrees south latitude. These air masses are cold and dense, so they have high pressure.
rill action
Rill erosion is a type of erosion that results in small, yet well defined streams. It happens when water from rainfall does not soak into the soil, but runs across it instead. The rills or small channels (often only 30cm deep) are caused when water running across the surface of the ground gathers in a natural depression in the soil, and erosion is concentrated as the water flows through the depression.
rill
Rills are narrow and shallow channels which are eroded into unprotected soil by hillslope runoff. Since soil is regularly left bare during agricultural operations, rills may form on farmland during these vulnerable periods. Rills may also form when bare soil is left exposed following deforestation, or during construction activities. Rills are fairly easily visible when first incised, so they are often the first indication of an ongoing erosion
high index Rossby waves
Rossby waves contract towards the pole and become straight, reduces north-south transfer of energy, and in the middle latitudes margins the climate is mild
clastic sedimentary rock
Sedimentary rock that forms when fragments of preexisting rocks are compacted or cemented together.
humic acid
The acid produced as water dissolves decaying plant material (humus)
percolation
The downward movement of water through soil and rock due to gravity.
felspar
The most abundant silicate, it weathers into clay
rainsplash erosion
The process of disturbing of the soil surface by the direct force of falling rain drops
surface wash
The process which occurs when infiltration capacity is exceeded or saturation is reached. Gullies are sometimes formed. Also occurs when ground is frozen.
fold mountain rock includes
a considerable thickness of sedimentary rock that has been crumpled and folded, suggesting the long fold mountain chains might mark the closure of former oceans
spit
a depositional structure projecting out into the water. spits are usually formed where the shoreline undergoes a sharp change of direction at a bay or estuary. longshore drift extends the beach out into open water.
turbidity current
a downslope movement of dense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension
graben
a dropped block of crust
alluvial fan
a fan-shaped mass of material deposited by a stream when the slope of the land decreases sharply
berm
a flat strip of land, raised bank, or terrace bordering a river or canal or beach.
advection fog
a fog formed when warm, moist air is blown over a cool surface and is chilled by contact. Typically this occurs over sea areas in early summer, when warm winds blowing polewards from tropical areas pass over the cool waters of higher latitudes, creating sea fog. With fairly light winds the fog forms close to the water surface, but stronger turbulence may lift the condensed layer to form a low stratus sheet. Areas susceptible to widespread sea god of this type include sea areas around north-west Europe.
fault
a fracture in rock along which rocks have been relatively displaced
doldrums
a frequently windless area near the Equator
occluded front
a front where a warm air mass is caught between two colder air masses and brings cool temperatures and large amounts of rain and snow
warm front
a front where warm air moves over cold air and brings drizzly rain and then are followed by warm and clear weather cold air is replaced by warmer air
piedmont
a gentle slope leading from the base of mountains to a region of flat land.
geosyncline
a geological depression of sedimentary rocks
how is hail formed?
a hailstone is composed of alternate concentric rings of clear and opaque ice, and is formed by being carried up and down into the vertical currents of a large cumulonimbus cloud. freezing and partial melting may occur several times before the hailstone can become large enough to escape the cloud.
duricrust
a hard mineral crust formed at or near the surface of soil in semi-arid regions by the evaporation of groundwater. - act as hard beds in the landscape, forming resistant cappings where valley incision has occurred. - formed by local soil weathering in situ or by the percolation of mineralised water
tor
a high rock, a high rocky hill, or pile of rocks, feature of granite landscape
thermal wind
a horizontal change in temperature causes wind speed to increase with height
sheet joint
a large fracture more or less parallel to a rock surface resulting from pressure release
offshore bar
a long narrow ridge of deposited material and sand, which can be found lying away from and parallel to a coast.
scarp
a long steep slope or cliff at the edge of a plateau or ridge. scarps are generally formed by one or both: differential erosion of sedimentary rocks, or by vertical movement of the Earth's crust along a geologic fault.
current
a mass movement of water
stream capacity
a measure of the maximum load of material a stream can move
negative feedback
a mechanism of response in which a stimulus initiates reactions that reduce the stimulus
block mountains
a mountain formed by natural faults in the earth's crust, separated by intervening basins
sheetflood
a moving sheet of water over the whole desert surface, generally occur in arid and semi-arid environments, although some claim are purely desert phenomena
Inter-Tropical Convergence Zone (ITCZ)
a narrow zone of low pressure near the Equator where northern and southern air masses converge
atoll
a ring-shaped reef, island, or chain of islands formed of coral.
conglomerate rock
a sedimentary rock made from smalled rounded stones that have been cemented together
salt pan
a shallow container or depression in the ground in which salt water evaporates to leave a deposit of salt.
laminar flow
a smooth pattern of flow, rare in natural streams but common in groundwater movement
tombola
a spit connecting an island to the mainland
what is a thunderstorm made of?
a storm frequently consists of several convective cells, each characterised by an updraft and going through three distinct stages.
fog
a stratus cloud that forms when air is cooled to its dew point near the ground
yardang
a streamlined rock structure formed by deflation and abrasion; appears elongated and aligned with the most effective wind direction. parallel U-shaped troughs separated by sharp ridges
what makes a monsoon?
a summer low-pressure cell develops over land because of thermal overheating, and this induces moist air to blow inwards from ocean seas
geostrophic wind
a theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.
flysch
a thick and extensive deposit largely of sandstone that is formed in a geosyncline adjacent to a rising mountain belt. especially common in the Alpine region of Europe.
igneous rock
a type of rock that forms from the cooling of molten rock at or below the surface
pediment
a very gently sloping inclined bedrock surface, typically slopes down from the base of a steeper retreating desert cliff or escarpment, caused by erosion
river delta
a wide, alluvial mouth of a river
mean sea-level air pressure around the world is
about 1013 millibars (Mb)
What does the ozone layer do?
absorbs UV radiation
kata-front
air is not vertically active, the general sinking of warm air at the front suppresses weather activity
because air is extremely compressible
air pressure decreases with height, with lowest layers of air being the densest
unstable air
air that does not resist vertical displacement; if it is lifted, its temperature will not cool as rapidly as the surrounding environment, and so it will continue to rise on its own. - environmental lapse rate exceeds adiabatic lapse rate
stable air
air that resists vertical displacement; if it is lifted, adiabatic cooling will cause its temperature to be lower than the surrounding environment; it will sink to its original position if physically possible
reworking of sand by long shore drift
aligns bayhead beach at right angles to the dominant breaker waves, for example Chesil Beach, Dorset.
joints
all rocks develop joints as they consolidate and crack under the stress set up by cooling or pressure changes
wave clouds
also called lenticular clouds are formed by stable air traveling in waves on the leeward side of a mountain, show the influence of topography on the flow of air.
middle clouds
altostratus, altocumulus
continental air mass
an air mass that forms over land; it is normally relatively dry
an anticyclone is
an area of high pressure, with roughly concentric isobars and a clockwise circulation of winds; it is the metrological opposite of a depression
karst topography
an area that has a land surface or topography with numerous depressions called sinkholes, feature of limestone landscape
horst
an elongated, uplifted block of crust bounded by faults
anticline
an upward fold in rock formed by compression of Earth's crust
what kind of warm fronts does the British Isles receive?
ana-front type. the rising motion in the air mass proceeds at different rates at different levels, and combined with varying relative humidity, produces a multi-layered effect rather than a solid mass of cloud. air motion is predominantly downwards. the relative humidity of cold air mass is low.
denudation/degradation
any process that wears away or rearranges landforms
large barrier beaches, for example those on the East Coast of the United States
appear to have been piled up parallel to the shore by the postglacial rise of sea level. several of these structures appear to be migrating landward and may eventually become part of a bayhead beach.
on the surface rocks
are brittle and fracture when subjected to stress and pressure, causing faulting
dark coloured minerals
are more susceptible to chemical weathering than light coloured minerals
more deeply buried rocks
are subject to higher temperatures and pressure, making them relatively plastic and so may respond to stress by folding rather than faulting
it is thought that many of the main English rivers such as the Trent and the Thames
are superimposed, as they cut across varied geology
maximum width of an abrasion platform
around 500m wide
moist/saturated adiabatic rate
as parcel moves upwards, its temperature drops at the dry adiabatic rate but the dew point temperature falls with the dew point lapse rate. as the rising process continues the air is cooled to dew point temperature and condensation starts to occur. If the parcel of saturated air continues to rise, latent heat release occurs. First, the uplifted air is being cooled by the reduction in atmospheric pressure. Second, it is being warmed by the release of latent heat from condensation. Which effect is stronger? As it turns out, the cooling effect is stronger, so the air will continue to cool as it is uplifted. However, because of the release of latent heat, the cooling will occur at a lesser rate. This cooling rate for saturated air is called the moist adiabatic lapse rate and ranges between 4 and 9°C per 1000 m (2.2-4.9°F per 1000 ft).
pediplanation
as scarps retreat over geological time pediments migrate and extend over large areas. The result is that the surface is eroded chiefly backward and that downward erosion is limited
dissipating stage of thunderstorm
as the supply of moisture in the cell is gradually exhausted, the energy reduces and the storm passes into this stage. this is characterised by downdraughts which spread out below the cloud, preventing further convective instability in the immediate vicinity.
radiation fog
associated with radiation cooling of the ground at night, where the ground then chills adjacent layers of air by conduction. Most likely to form under fairly calm conditions with clear skies, which allow maximum outgoing radiation. Autumn and early winter are usually seasons most prone to radiation fog. Light turbulence will transport cooler air upwards and promote a thicker fog layer. The fog usually evaporates fairly rapidly after sunrise, once incoming radiation begins to warm the ground.
when offshore bars are long and continuous they create
barrier beaches
igneous group
basic rocks such as basalt (contains felspar) weather much more rapidly than acid rocks such as granite
winds tend to increase with altitude
because of lower air density
river gradients tend to
become less steep downstream as the load gradually abraded itself and becomes easier to support, eroding the channel bed less
subtropical high pressure belts
belts of persistent high atmospheric pressure trending east-west and centered on about 30 degrees N and S. in the northern hemisphere, the belt is more discontinuous by land masses, and high pressure usually occurs only over the ocean areas as discrete cells (Azore and Hawaiian cells).
magnetosphere
beyond thermosphere and exosphere, where the influence of Earth's magnetic field is felt but there is no atmosphere
the longest lasting anticyclones occur when the upper air flow adopts a low zonal index, breaking down into a cellular pattern
blocking anticyclones may develop, which block the passages of surface depressions
Equatorial westerlies
blow between the trade winds at certain times of year, occur in summer in each hemisphere when trade wind belts have moved poleward. particularly noticeable over the low latitude land areas of the northern hemisphere. the south-west monsoon of India is an exaggerated expression of these winds.
in winter, trade winds
blow from an easterly quarter, often having crossed hot deserts, and bring dry conditions (eg, Nigeria) OR having crossed large tracts of warm water (eg, the Carribbean) where the winter season is moister
rising pressure usually suggests
brighter weather
in Africa, the south-west monsoon
brings precipitation in June to a large tract of the country stretching from the savanna lands of West Africa eastwards to Ethiopia. Precipitation reaches a maximum on the western edge of the Ethiopian mountains.
fronts
broad mixing zones in the atmosphere where horizontal gradients of pressure, temperature and other properties of air masses become steepened
when basal weathering surface revealed
called an etchplain because of etched surface
westerlies in both hemispheres and cyclone tracks
cause high rainfall in the middle latitudes
regions of subsiding air, which is warmed adiabatically and made dry
cause low rainfall in the middle latitudes, mainly in the subtropics on the eastern sides of the ocean (eg the Sahara)
Coriolis effect
causes moving air and water (and all free moving objects) to turn left in the southern hemisphere and turn right in the northern hemisphere due to Earth's rotational deflecting force. nearer the equator, where earth's surface is spinning on a plane almost parallel to the axis of rotational, the Coriolis effect is very slight. in higher latitudes it has more marked effects.
hydration
certain types of mineral expand as they take up water, causing additional stresses in the rock. For example, gypsum (calcium sulphate) becomes anhydrate. Technically mechanical but usually occurs alongside hydrolysis
what makes relative humidity change?
change in temperature which changes capacity of air to hold water vapour, and change in water content
swell environment
characterised by flat constructional waves
dominant weathering process in humid tropical areas
chemical weathering
dominant weathering process in humid temperate areas
chemical weathering, except possibly at elevations above 600, where freeze-thaw action is important
dominant weathering process in deserts
chemical/moisture-related weathering
high clouds
cirrus, cirrostratus, cirrocumulus
kata-cold front
cloud development is limited and dominated by thick stratus clouds. the front passes through almost unnoticed with only gradual changes in temperature, pressure, wind and precipitation amounts.
moisture and temperature
cold air can hold very little moisture, and consequently the water vapour content is always low in absolute terms when the temperature is low. as the temperature increases, the saturation limit also increases: air at 20 degrees can hold four times as much water by weight as air at freezing point.
polar easterlies
cold winds that blow from the east to the west near the North Pole and South Pole
what causes the high pressure of an anticyclone?
cold, dense air somewhere in the system
in the monsoon season, a lot of the rain
comes from inconsistent migrating cyclonic disturbances, mostly from the Bay of Bengal, which makes the precipitation highly variable from year to year
condensation nuclei
common salt from the sea, dust, and pollution particles
dissolved load
composed of soluble materials transported in the form of ions from chemical weathering
abrasion in desert (wind erosion)
confined to the saltation zone, a metre off the ground. wind abrasion is responsible for undercutting and fluting rocks, polishing hard rock surfaces and shaping stones into faceted forms.
how is air induced to rise?
convection from heating of ground below, orographic uplift over hills and mountains, turbulence in air flow and uplift at frontal surfaces
horizontal net flow into a region of air (upper air movement)
convergence
compared to depressions, anticyclones usually
cover a wider area and tend to be more persistent and slow-moving
tidal currents
created as the tide ebbs and flows and are usually powerful enough to move the sediments in estuaries. the ebb tide is usually stronger as it is reinforced by the outflow of river water.
how are sand surfaces regularised?
creates regular ripples - surfaces facing wind receive more bombardment than lee slopes - creeps up to peaks of ripples faster than sediment fills hollows - crest grows and hollows deepen - an optimum ripple amplitude is developed in equilibrium with the prevailing wind speed
hydrolysis
crystalline felspathic rocks + water -> clay, sand and carbonates in solution, decomposition and disintegration, process accelerated by presence of carbonic acid in rainwater
clouds with vertical development
cumulus, cumulonimbus
channel roughness
decreases downstream
deflation
deflation occurs with the initial removal of weathered sand.
blowout dune
depression excavated by wind in easily eroded sediment
storm wave environment
destructive storm waves frequent and shingle beaches are common
cyclogenesis
development or strengthening of a mid-latitude cyclone (frontal depression)
air over mountains
diagram
geostrophic and surface wind
diagram
phase changes
diagram
thermal winds
differing vertical temperature gradient in two columns create an increasing pressure gradient
ana-cold front
distribution of clouds and precipitation similar to that of an ana-warm front, but in reverse. the frontal slope is steeper, so the belt of cloud and rain passes over more quickly, creating a more abrupt change in weather.
horizontal net outflow from a region of air (upper air movement)
divergence
documentary evidence
documentary evidence is any evidence that is, or can be, introduced in the form of documents, as distinguished from oral testimony
strong convergence of air takes place
downstream of the ridges in the Rossby pattern, and here anticyclones or ridges of high pressure may persist at the surface level
pressure gradient force
drives air from areas of higher pressure to areas of lower pressure
parabolic dunes
dunes: vegetation partially covers land, on coastlines, tips point into wind, strong winds, abundant sand opposite to Barchan form because vegetation impedes migration downwind of the lower slopes of the dune
low energy coasts tend to have landforms such as
estuaries, spits and bars, deep embankments, finer sediment, mud flats
artic air mass
extremely cold air mass that originated in the far north
silica is
extremely durable, as shown in the case of quartz and flint
positive feedback
feedback that tends to magnify a process or increase its output.
Eustatic change
fluctuations in sea level associated with adjustments in the hydrologic cycle, associated with the growth and wave of ice sheets
where does potentially unstable air occur?
for example, at a frontal surface or in air approaching higher ground. It is fairly common among warm air masses which have picked up a good deal of moisture in their lower layers on passing over sea areas. Generally in air masses which are moist in their lower layers and dry in their upper layers.
epeirogeny
forces acting along a radius from the earth's centre to the surface, characterised by large-scale uplift or submergence of land areas. -slow and widespread so no obvious folding or fracturing produced in rocks.
orogeny
forces acting at a tangent to the surface, like those involved in plate tectonics. created fold mountain ranges, complex fold structures - in contrast to epeirogeny, an orogeny tends to occur during a relatively short time in linear belts and results in intensive deformation. - usually accompanied by folding and faulting of strata, development of angular unconformities and the deposition of clastic wedges
orographic depressions
form when the flow of an air stream is affected by a mountain barrier, causing a piling up and divergence of air on the windward side, and a relative deficit and convergence on the Lee side. British mountains are not high enough to show an example of this, but they are common in Europe.
aggregational
formed of aggregate rocks etc
equatorial air mass
formed on the equator, very warm
cap clouds
formed when an uplifted moist airstream reaches condensation level only at the very summit, where a small cloud forms. further downwind in the lee of the hill the air sinks again and the cloud dissipates. stationary and found over mountains.
cold front
forms when cold air moves under warm air which is less dense and pushes air up (produces thunderstorms heavy rain or snow) warm air is replaced by colder air
the biggest coastal dunes are
found on west-facing coasts in temperate storm belts of the world.
dominant weathering process in cold regions
freeze-thaw weathering
occulusion
frontal depression is raised as cold front travels faster than the warm and overtakes it, creating a new front
rime
frosty coating, like dew
why is the sky blue?
gas molecules and dust particles in the atmosphere scatter radiation more efficiently at low wavelength, so we see the sky as blue because it's at the lower wavelength end of the light spectrum
steam fog
generally more localised than the other two main types, likely to develop in situations where cold air blows over much warmer waters. Evaporation from the water body quickly saturates the cold air and the resulting condensation is seen as steaming. The best examples occur in polar areas, where a great contrast in temperature can exist between the very cold air that blows off ice covered land and the relatively warm surrounding seas. In north polar regions such fog is sometimes called Arctic Sea Smog.
clays and shales are
generally not susceptible to chemical weathering, except though impurities they contain such as ferrous oxides
in Britain, what accounts for many of the sea levels changes at the coast?
glacial eustasy and glacial isostacy
lag in temperature change
greater over oceans than continent et because land has a low specific heat capacity, while oceans have a higher one and also store heat by transporting it downwards in currents to mix with deeper waters. there is a lag because it takes time for air temperature to rise after ground has been heated by solar insolation/drop after ground receives less solar insolation
corals theoretically cannot
grow in muddy water, or water any deeper than 50m
hypothesis of rotting and stripping in the formation of bornhardts
has also been applied to the formation of tors in England; the deep rotting episode took place when England experienced a warmer climate.
the frictional drag exerted on the airflow above
has an effect on the balancing of other wind forces. friction lessens the speed of the wind, which lessens the Coriolis force. This allows the pressure gradient to assert greater strength by causing the air to flow more towards low pressure. The usual result is that surface winds flow at a slight angle to the isobar. the effect is strongest in calm conditions at night.
latent heat
heat absorbed or radiated during a change of phase at a constant temperature and pressure
moist rocks
help to promote chemical weathering - in the British isles, rocks are moist because of low evaporation rates - in deserts, rocks are moist because some water vapour is deposited as dew
the solar constant
how much radiation the earth receives from the sun, varies little in intensity from year to year but marginally affected by sunspots and variations in the Earth's cycle
tropical depressions
hurricanes, cyclones, typhoons.
cavitation
hydraulic action from pressure under high velocity flow
evorsion
hydraulic action from the direct impact of water
adiabatic principle
if no energy is added to a gas, its temperature will increase as it is compressed. when a gas expands, its temperature drops by the same principle. - adiabatic process refers to a heating or cooling process that occurs solely as a result of pressure change, with no heat flowing into or away from a volume of air.
igneous rocks joint
igneous rocks develop columnar joints in response to contraction when cooling (eg, basalt)
potential instability (convective instability)
in air masses which are moist in their lower layers but dry in their upper layers. The air may initially be stable, but on lifting, the dry upper parts will rise dry-adiabatically whereas the lower moister parts quickly become saturated on rising and cool at the saturated adiabatic rate. With different parts of the air cooling at different rates, this may change the temperature distribution throughout the mass, transforming a stable situation into an unstable one.
asymmetrical channel
in an asymmetrical channel, the zone of maximum velocity shifts away from the centre towards the deeper side, so which causes the zone of maximum turbulence to become lower on the deeper side - this means significant erosional effects are brought about by change in channel shape
selective deep weathering and bornhardts
in areas of rotted rocks there are two denudation surface: ground and the basal weathering surface (plane underground, between rotten and non-rotten rock) - basal is irregular and complex surface, deepest where strongly jointed rock - some unrotted round boulders on plane called corestones - subsurface domes. similar in shape to exposed bornhardts so suggests connection in origin. selective subsurface weathering rots rock around subsurface dome and then rotted rock is removed. - curved shape means water runoff, causing more erosion and more doming
currents are more effective
in shaping sea floor topography than beaches, and waves are more effective in shaping beaches than currents
wadi
in the desert, a streambed that is dry except during a heavy rain, channel characteristically flat-bottomed and steep-sided
how can new thunderstorm cells form?
in the dissipating stage, new cells may be initiated by the meeting of cold downdraughts from cells a few metres apart, triggering the rise of warm air in between.
previous role of water in shaping desert geomorphology
in the driest regions of, for example, the Sahara, there are features such as gullied mountains and deep wadi systems that are too extensive to be explained by the present level of activity of running water. This suggests that water was more important as a landscape former in the past, even if topographic features are undergoing relatively little modification today - archaeological and documentary evidence which confirms past occurrence of wetter phases in the Sahara
geostrophic wind
in the free atmosphere, above the level of air flow affected by surface topography, winds generally blow at right angles to the pressure gradient. this indicates that the pressure gradient force is exactly balanced by the Coriolis effect acting in an opposite direction.
Buy Ballot's Law
in the northern hemisphere, if one stands with their back to the wind, low pressure always lies on the left and high pressure to the right. the reverse applies in the Southern Hemisphere.
why does evaporation reduce temperature?
in the process of evaporation, energy in the form of heat is used to change the water from a liquid to a vapour. the loss of heat energy cools the air, but that loss is then compensated by conduction and radiation.
when is the Rossby wave pattern most noticeable?
in winter, when the westerly circulation is strongest. a breakdown of the Rossby flow can lead to persistent cold anticyclones which bring cold air to some lower margins.
discharge, channel width, depth and velocity
increase downstream
low index Rossby waves
increases amplitude of waves, decreases wavelength, wave system breaks down into a (swirly) cellular pattern. this means more cold air can move southwards in the troughs of the cellular pattern while more hot air can move northwards; maximum poleward transport of energy in the mid-latitude circulation.
developing stage of thunderstorm
initial updraft is formed in response to the uplifting mechanism. the draught is considerably accelerated by the energy provided by the release of latent heat as condensation occurs. the whole cloud becomes rapidly out of thermal equilibrium with its surroundings. precipitation processes begin as the cloud develops beyond the freezing level, but the great strength of the updrafts initially prevents the snow and ice from falling.
bornhardt
inselberg made of highly resistant rock such as granite or gneiss (resistant to chemical weathering). characteristically dome shaped with precipitous sides, large areas of bare rock surface and an absence of talus at foot. show extensive sheet jointing very large bornhardts are described as sugar loaves (eg Sugarloaf mountain Rio de Janerio)
angular unconformities
interruptions in the normal deposition of sedimentary rock
spherodial weathering
intrusive igneous rocks tend to develop regular horizontal and vertical joints, creating angular blocks which become rounded during the course of weathering in a process known as spheroidal weathering
diabatic temperature changes
involve the physical mixing of air, unlike adiabatic ones which occur without physical mixing
reflected radiation from the Earth's surface
is long-wave radiation rather than short, and so is absorbed better by atmospheric gases rather than escaping into space. the atmosphere then reflects the radiation back to the ground creating the greenhouse effect.
around 95% of kinetic energy provided by the weight and elevation of the water
is lost from friction with the channel
in the humid tropics rock decomposition through chemical weathering
is more rapid that than the movement of material on slopes, which is also more important than river processes
the ideal form of channel for the discharge of water
is semicircular in cross section
inselbergs
isolated, steep-sided erosional remnants that rise above terrain
how do joint directions influence landform alignment?
joint directions affect the erosional processes. For example, in many valley floors, streams follow joint directions, particularly in igneous and sedimentary rocks.
variation of relief and structure in deserts
large dune areas occupy only 10 percent of the Sahara. many other areas are mountainous, or characterised by gravel plains which may have some thin scrubby vegetation in the semi-arid margins
why are bornhardts domed?
large sheet joints are curved, developed by a pressure-release mechanism
sand ridges
larger than ripples and made of coarser material, too large to be lifted by wind material shaped into ridges by the impact of grains undergoing saltation, which can move coarse material up to six times their own size
pressure-release jointing
layer after layer of rock peels off in curved slabs or plates, thinner at the top of the rock structure and thicker at the sides
in the northern hemisphere cold air always lies to the
left of the thermal wind when viewed downwind
protected environment (waves)
low energy conditions of enclosed or partially enclosed seas
zone of saturation
lower soil layers where all spaces and rocks are filled with water
antecedent river
maintains its original course and pattern despite the changes in underlying rock topography
map of different wave environments
map
where do fronts tend to form?
margins of oceans and continents, and the ice margins of high latitudes
absolute humidity
mass of water vapor in a given volume of air, direct way of expressing humidity, grams per cubic metre
stream competence
measuring the maximum size of the sediments transported by a river
where are the westerlies?
mid latitude, develop out of the poleward sides of the subtropical high pressure cells at between 30-60 N and S
where is the highest velocity in a river?
midstream about a third of the distance down from the surface to the basin
cliff bevel
might be because of weathering and erosion; bevels are often associated with fossil scree deposits and so could be the result of past glacial action
atmosphere has several
millions of tons of mass and weight, which exerts pressure of about 1kg/square centimetre at sea level
difference between moist and dry adiabatic lapse
moist happens higher in the atmosphere, after latent heat release and after cloud formation. the cooling effect is stronger higher in the atmosphere, so air will continue to cool as it is uplifted. But due to latent heat release, cooling is slower. The slower cooling of saturated air is called the moist adiabatic lapse rate. Unlike the dry adiabatic lapse rate, which remains constant, the moist adiabatic lapse rate is variable because it depends on the temperature and pressure of the air and its moisture content.
Ozone is a gas consisting of
molecules containing three oxygen atoms.
ionosphere (same level as thermosphere)
molecules of gas become ionised, and the electrically charged molecules in the thermosphere absorb radio waves during the day and reflects them back at night.
seif dunes
more sinuous crest and are usually shorter than longitudinal dunes. long and straight, often occuring in sub-parallel lines.
why are tropical rivers inefficient at erosion?
most coarse debris from landslides moving downslope is subjected to weathering and only very fine material reaches the stream. there is no pebble sized material in the load to erode with. tropical rivers often fall over sedimentary structures of crystalline masses, and have seasonal floods rather than the irregular floods of temperate rivers that make better at eroding.
jet streams
narrow bands of high-speed winds that circle the earth, blowing from west to east intense thermal winds associated with latitudes where the poleward temperature gradient is particularly strong
why can't pressure gradient and the Coriolis effect be the only effects on wind?
neither the pressure gradient force nor the Coriolis effect can account for the fact that global winds generally blow with steady force and direction. - if global pressure gradient were to act alone it would cause winds to blow directly down the gradient with increasing acceleration - if the Coriolis effect acted alone, winds would eventually blow in a circle.
temperature inversion
occurs in the troposphere when temperature increases as altitude increases, as the ground cools the lower layer of air by conduction
technically evaporation
occurs when the vapour pressure at a water surface exceeds that of the atmosphere above
when do absolutely unstable air conditions occur?
on very hot days, when the ground layers of air are considerably heated, giving a higher lapse rate. if the air is moist enough, strong vertical cloud development is likely.
adiabatic cooling
parcel of air is uplifted, atmospheric pressure on the parcel becomes lower and the air expands and cools.
water-layer weathering (around the coast of Australia and other subtropical latitudes)
platforms are created by this process at or above the spray zone, where alternate wetting or drying of the rock leads to its gradual disintegration. a further bench is formed when storm waves sweep away the weathered debris. - however, it has been suggested that these platforms may be partly old abrasion platforms formed at a higher sea level
falling pressure usually suggests
poor weather
why do braided channel patterns occur?
possibly because the stream doesn't have the capacity to transport its load in a single channel
saturation vapour pressure
pressure exerted by water vapour when the air is saturated
westerlies
prevailing winds that blow from west to east between 30 degrees and 60 degrees latitude in both hemispheres
carbonation
process by which minerals reacts with carbonic acid (water and carbon dioxide) to form compounds such as calcium bicarbonate which are soluble and can be carried away in groundwater
isostatic adjustment
process of establishing a new level of gravitational equilibrium when earth's crust makes up and down movement to adjust tectonic shifts in load
moisture in deserts
promotes chemical weathering
the degree of high pressure in an anticyclone
rarely matches the intensity of low pressure in a depression
relative humidity
ratio of the actual amount of water vapour in the air to the maximum amount the air could hold at that temperature, expressed as a percentage
hydraulic radius
ratio of the cross-sectional area of the channel and the length of its wetted perimeter. - the higher the ratio the more efficient the stream and the smaller the loss due to friction
specific humidity
ratio of the weight of water vapour (grams) to the weight of moist air (kilograms).
koppies
rectangular inselbergs
spits are usually
recurved at their extremities, the recurves being produced by wave refraction around the head of the spit, or wave approach from a different direction, or the prevailing wind direction
warm anticyclones
relatively warm air in the lower parts of the troposphere. the excess of pressure comes from the cold air in the upper troposphere and lower stratosphere. this is how permanent subtropical anticyclones are formed.
on a global scale, the gradual poleward decrease of temperature in the atmosphere from the equator
results in a large westerly component in the upper winds; in both hemispheres most upper winds are westerly
in the Southern Hemisphere cold air always lies to the
right of the thermal wind when viewed downwind
the rate of most chemical reactions
rises with increased temperature and moisture
superimposed river
river changes it course to take an expected route over rock topography
intrusive igneous rock
rock formed from the cooling and solidification of magma beneath Earth's surface
extrusive igneous rock
rock that forms from the cooling and solidification of lava AT Earth's surface
metamorphic rock
rock that has been changed by heat and pressure
which process is most important for creating dunes?
saltation is very significant in building dunes as turbulent wind flow near the ground creates a variety of short, steeply sloping ascents. the impact of this grain against the ground sand may cause other particles to move by creep.
salt crystallisation weathering
salts such as sodium chloride enter rocks in a dissolved form, then dry and crystallise, disrupting the rock. This produces cavernous weathering, creating the small-scale honey combing of rock surfaces
rotten rock
saprolite (rotten rock) is a chemically weathered rock representing deep weathering of the bedrock surface. In most outcrops its colour comes from ferric compounds. Deeply weathered profiles are widespread on the continental landmasses between latitudes 35°N and 35°S. Conditions for the formation include: - a topographically moderate relief flat enough to prevent erosion and to allow leaching of the products of chemical weathering. - long periods of tectonic stability; tectonic activity and climate change can cause erosion. - humid tropical to temperate climate. Poorly weathered saprolite grit aquifers are capable of producing groundwater, often suitable for livestock. Deep weathering causes the formation of many secondary and supergene ores - bauxite, iron ores, saprolitic gold, supergene copper, uranium and heavy minerals in residual accumulations.
sedimentary rock joint
sedimentary rocks usually develop joints at right angles to their bedding Pläne
thermal depression
shallow low pressure areas develop because of overheating of the earth's surface. small diurnal examples may occur over islands and peninsulas in summer and lakes and seas in winter. continental low pressure cells develop in summer over the large arid regions of the world, for example central Australia and the South-west United States.
bahada
slope (aggregational feature)
depositional desert features
small scale sand ripples and ridges, and large scale dunes
carbon dioxide is twice as soluble at 0 degrees than 20 degrees
so chemical weathering is also important in cold regions
Barchan Dunes
solitary, crescent-shaped, tips point downwind, limited sand supply, little vegetation, constant wind directions
low clouds
stratus, nimbostratus, stratocumulus
what may orogenies result from?
subduction, landmass expansion, plate overriding or underthrusting, continental collisions
when do sublimation and deposition occur?
sublimation occurs when snow patches disappear without meting, and deposition of ice from vapour in the form of rime is quite common in very cold climates and at high altitudes
the only areas where surface pressure patterns persist to high levels are
subtropical regions, where high pressure cells still exist with height; therefore these latitudes have a key role to play in the general circulation of the atmosphere.
weather front means change in
temperature and wind
what is the most important factor for variations in atmospheric moisture content?
temperature, because it fundamentally affects the saturation limit of the air
channels in silt and clay
tend to be deeper and narrow than those in sand and gravel, because the finer materials are cohesive and promote bank stability
occulusions reaching the British isles
tend to be of the warmer type in winter, and cold in summer. they create some of our wettest frontal weather.
marked convergence and divergence in the upper atmosphere usually take place when
the Rossby wave pattern is weak or has broken down
wetted perimeter
the area where the river is in contact with the channel
normal lapse rate
the average rate of temperature decrease with increasing altitude in the lower atmosphere, about 6.4 degrees per kilometre
when studying hydrology of rivers
the basin hydrological cycle is used, where the basin is the focus, rather than the global hydrological cycle
lag deposit
the coarse sediment left behind in a desert after wind erosion removes the finer sediment
cold anticyclones
the cold air is confined to the lowest parts of the atmosphere, caused by the cooling of the earth's surface in winter. this chills the adjacent air, causing it to contract and initiate the subsidence above. - eg, seasonal anticyclones over Siberia and Canada in winter
over Siberia and Canada
the coldness of the land chills the air to cause the development of large high pressure cells over these regions
what increases evaporation?
the drier the air, the greater the evaporation; increased temperatures; greater the wind strength, more effective the evaporation is, because in turbulent conditions saturated air is continually replaced by fresh air
diastrophism
the earth force that folds, faults, twists, and compresses rock in the crust
atmosphere carries solid materials in suspension, which take the form of minute dust particles from natural causes or from manmade pollution
the fine material forms the necessary nuclei on which water vapour can condense to form water droplets and eventually precipitation
why does condensation increase temperature?
the heat loss during evaporation passes into the water vapour in the form of latent heat. when the reverse process condensation takes place, the latent heat locked in the water vapour is released back to the atmosphere, causing a slight rise in air temperature.
tropopause
the interface between the troposphere and the stratosphere, acts as a ceiling to weather generated in the troposphere, height varies seasonally and latitudinally, higher in elevation in summer than winter and higher at the equator than the poles
critical erosion velocity
the lowest velocity at which grains of a certain size can be eroded
mature stage of thunderstorm
the mature stage is reached with the sudden onset of heavy rain, perhaps accompanied by thunder and lightning. the precipitation drags cold air down with it which is often felt on the ground. during the latter phase of the mature stage, the upper parts of the cloud composed of ice particles reach the tropopause. here the cloud spreads out into a characteristic anvil shape, which might be elongated by high altitude winds.
glacial isostatic adjustment
the ongoing movement of land once burdened by ice-age glaciers
the difference between coastal dunes and desert dunes is
the presence of vegetation, in Britain mostly sea-couch grass and marram grass, which has a trapping effect on blowing sand.
water vapour pressure (millibars)
the pressure at which water vapour is in thermodynamic equilibrium with its condensed state.
in the summer in either hemisphere, the poleward migration of the equatorial trough brings weather conditions similar to those of the equatorial rainy zone.
the rainy season usually begins and ends with heavy thunderstorms associated with the unstable conditions of the passing intertropical convergence zone.
what causes the rapid rotation in tornadoes?
the rapid convergence of air at the base of the cumulonimbus cloud, drawn in as rapid updraughts develop.
dry adiabatic lapse rate
the rate at which a rising air parcel tray has not yet been copped to saturation changes temperature with change of height. - called the dry rare because no condensation occurs during this process
environmental lapse rate
the rate of temperature decrease with increasing height in the troposphere
post glacial rebound
the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression
backswamp
the section of a floodplain where deposits of fine silts and clays settle after a flood
lee side
the side of something that is sheltered from the wind
pedology
the study of soil
dew point temperature
the temperature to which air has to be cooled in order to reach saturation and begin condensing
why do coarse grained granites weather more quickly?
the weathering of one type of large crystal will have a proportionally larger effect in weathering the whole rock. additionally, fine grained granites often possess a better interlocking structure between their crystals.
wind shear
the wind at higher air layers is stronger than the wind at lower ones
why are rocks so chemically weathered in the tropics?
theory one: tropical conditions provide the optimum conditions of warmth and humidity for chemical weathering. large quantities of humic acid and more carbon dioxide. theory two: weathering proceeds slowly, and weathering is only so deep because it has continued undisturbed for a long time
why does the northern hemisphere have much greater and more complicated seasonal contrasts in its pressure distribution?
there is an unequal distribution of land and sea between the two hemispheres. ocean areas which dominate the southern hemisphere tend to be much more equable than continents in both temperature and pressure variations.
in the Carribbean and China sea, the summer rainy period is frequently punctuated by disturbances developing along the edge of the convergence zone and travelling westward.
these perturbations usually occur towards the end of summer and some of them grow into hurricanes
coral coasts are unique because
they are composed entirely of matter accumulated through organic processes
as air masses migrate from their place of origin
they gain the opposite characteristics, with cold air masses more subject to modification (warming up and becoming unstable) than warm air masses
ozone depletion
thinning of Earth's ozone layer caused by CFC's leaking into the air and reacting chemically with the ozone, breaking the ozone molocules apart
favoured areas for cyclogenesis are
those related to the main semi-permanent troughs in the upper air pattern, downstream of the Rockies and the Tibetan Plateau
slope material movement in rainforests
thought to be more active than on temperate hillsides - rain is heavier and more intense - ground flora is less well-developed in a tropical rainforest - rainforest humus layer is less well developed with a rapid rate of oxidation - rainsplash impact and surface wash probably more important on tropic forest floors than in temperate forests
Rossby waves
three to six waves visible in each hemisphere, naturally occur in rotating fluids, link features of general circulation in hemispheres
why does thunder occur?
thunder occurs because lightning hears the immediate air to very high temperatures, which causes rapid expansion and vibration of the air column heard by us as thunder.
in a surface low pressure system, there is a net influx of air, with the actual wind motion blowing at a slight angle to the isobars.
to maintain the low pressure, the influx has to be removed through vertical ascent, or the depression would fill up and disappear. the vertical motion of air is also accompanied by outward divergence at a high level.
why does lightning occur?
to relieve the electrical tension between oppositely charged areas, between the cloud and the ground or within the cloud. generally, the upper part is positive, lower part negative, and there is a small positively region around the rain area.
sandstones are susceptible
to weathering only in the material which cements the grains together, as these grains are often silica etc
layers of the atmosphere lowest to highest
troposphere, stratosphere, mesosphere, thermosphere
eddy currents
turbulent flow
how are seif dunes formed?
two theories: - roll vortices in the wind whip the sand into sharp ridges - alternatively, seif dunes require multidirectional wind patterns, and they develop as oblique features where a cross-wind elongates a pattern created by the prevailing wind
tornado
ultimate stage of cyclonic spin and energy. overhead them is a large cumulonimbus cloud and a funnel cloud projecting from its base to the ground. the great pressure fall in the whirl causes cooling and saturation, which makes the funnel cloud visible.
headward erosion
undercutting at the base of soil or rock, often caused by percolation underground, which removes fine material and develops subsurface pipes and tunnels. these become exposed as open gulleys as they widen and cave in.
zone of aeration
upper soil layers that hold both air and water
sedimentary consistently
vary in their resistance to weathering
friction in river
velocity distribution of river flow is not even, water flows more slowly near the banks and fastest in the centre
ana-front
vertical motion of air is rising rapidly and unstable, creating an active front
the retreat of the monsoon is associated with
violent storms in the bay of Bengal and the Arabian sea. Bangladesh has particularly suffered from these in recent years.
tropical air masses
warm air that form in the tropics and have low air pressure
supercooled water droplets
water droplets existing at temperatures below freezing, will relatively easily turn to ice crystals
flash floods in deserts
water run-off has dramatic erosional effects. because long periods of drought exist between the intermittent bouts of rainfall, weathering and wind action, there is a lot of loose surface debris. therefore, flash floods, caused by heavy conventional storms, pick up an enormous load to erode with. run-off will be great, with little vegetation, but streams will eventually suffer heavy percolation losses and choke on the debris
frontal depression
wave distortion creates a low pressure area between two fronts, the development of a wave in the warm front traps it in a warm depression between the cold air in front and behind
in anticyclonic conditions
weather conditions are very stable with only light winds or calm at the centre of the system
a coarse grained granite
weathers more quickly than a fine grained granite
some authors think that the main north-south rivers
were initiated on a covering layer of Pleistocene marine gravels, but now flow discordantly across east-west trending anticlines.
why are charges distributed like they are in a thundercloud?
when a supercooled water droplet freezes inward from its surface, it gains a positively charged outer core and a negatively charged warmer core. with rupturing during the freezing process, splinters from the outer surface are carried to the top of the cloud in draughts, leaving the lower part negatively charged. the positively charged part near the rain might be an induced charge from the ground, which is usually negative.
advantage of absolute humidity
when air is lifted to higher elevations without loss or gain of water, absolute humidity is affected by the volume change while specific humidity stays constant, because it's measured by weight not volume
centripetal force
when isobaric pattern is markedly curved, usually around cyclones or high pressure areas, can make the Coriolis effect stronger than pressure gradient in HIGH pressure areas (and weaker than in low pressure areas)
knickpoint
when longitudinal profile has an abrupt change in gradient
conditional stability
when moist air has an environmental lapse rate between the dry and wet adiabatic rate. it is stable in respect to the dry rate and would normally sink to its original level if displaced. However, if the air should become saturated, (for example being forced to rise to higher elevations), on rising by the saturated adiabatic rate it would become warmer than the environmental air and continue to rise of its own accord. Therefore the air is unstable, conditional on it being saturated.
thermodynamic equilibrium
when there are no net flows of matter or energy to or away from a system and no net changes in the matter and energy in that system.
deflation hollow
where deflation has been confined to a specific area (for example, if chemical weathering has been strong around a salt pan) a deflation hollow may be created. the deflation of larger areas leaves behind a gravel or stone strewn surface known as a hamada.
where do braided channels occur?
where river discharge is highly variable and the banks are most easily erodible, supplying an abundant bed load. therefore, braided channels tend to occur in semi-arid regions and in glacial areas.
in a surface high pressure system, there is a divergence of air at low levels
which is maintained by convergence higher up
discharge equation
width x depth x velocity
how does wind move sand?
wind moves sand in suspension, by surface creep and by saltation. the sand moved by these processes is uniform in size, between 0.1 to 0.5mm wide.
pressure gradient is strongest in
winter, when the temperature contrasts between the polar cells and the equator are most marked
when would a very wide sea platform have been created?
with steadily rising sea level
frontal zone slopes upward at a low angle (<1 degree)
with the colder of the two air masses forming a wedge underlying the warmer air mass; this is why you can see an approaching cold front before it reaches you
equatorial trough
zone of rising warm air and low pressure at the equator
